1. Field of the Invention
The present invention relates to semiconductor devices, and more particularly, to a trench top oxide (TTO) nitride liner for improved collar protection and TTO reliability in trench-capacitor vertical dynamic random access memory (DRAM) cell array devices.
2. Discussion of the Prior Art
A processing scheme employed in forming the above-described vertical DRAM cell arrays is described in detail in commonly-owned, co-pending U.S. patent application Ser. No. 09/777,576 entitled STRUCTURE AND METHOD FOR A COMPACT TRENCH-CAPACITOR DRAM CELL WITH BODY CONTACT the contents and disclosure of which are incorporated by reference as if fully set forth herein. Described now with reference to FIGS. 1(a)-1(f) are the various processing steps for forming vertical DRAM cell arrays. As shown in FIG. 1(a), there is depicted an initial structure that is employed in fabricating a vertical DRAM cell array. Specifically, FIG. 1(a) shows an array portion of the structure that includes Si-containing substrate 10 having a material stack comprising an etch stop pad layer 12 and a hard mask 14 formed thereon. The substrate may include well regions 11, or the well regions may be formed later in the process.
The structure shown in FIG. 1(a) is made from conventional materials that are well known to those skilled in the art and conventional processes also well known in the art are employed in fabricating the same. For example, etch stop pad layer 12 may be composed of an oxide such as SiO2 and is formed on a surface of Si-containing substrate utilizing a conventional thermal growing process and thereafter hard mask 14, e.g., SiN, is formed on etch stop pad layer 12 by a conventional deposition process such as chemical vapor deposition (CVD), plasma-assisted CVD, sputtering, or chemical solution deposition. It should be understood that, additionally, a thick oxide layer may be formed over the SiN layer to serve as an improved hard mask during subsequent trench etching. Further, the thickness of each layer of the material stack may vary. Typically, however, the etch stop pad layer has a thickness of about 5 nm or less and the hard mask has a thickness of from about 10 to about 1000 nm.
Next, deep trenches 16 are formed in the structure shown in FIG. 1(a) providing the structure shown in FIG. 1(b). The term xe2x80x9cdeep trenchxe2x80x9d is used herein to denote a trench whose depth from the top surface of Si-containing substrate 10 is from about 1.0 xcexcm or greater. The deep trenches are formed in the conventional manner of opening a trench pattern extending through the hard mask, the etch stop pad layer and a portion of the Si-containing substrate via lithography and anisotropic etching. Note deep trenches 16, which are formed in rows and columns into the Si-containing substrate, are the areas in which the storage capacitor and vertical MOSFETs are formed.
A polysilicon buffered LOCOS (local oxidation of silicon) collar or other like collar oxide 18 is then formed in a portion of the deep trench, See FIG. 1(c). Next, a capacitor (not shown in the drawings) is formed in the lower portion of the deep trench utilizing conventional processing steps well known to those skilled in the art. Included in the deep trench-processing steps is the formation of a buried plate diffusion region (not shown) and formation of node dielectric 20 about said buried plate diffusion region. As shown in FIG. 1(c), node dielectric 20 extends from the lower portion of the deep trench to the upper surface of the deep trench and is formed on walls thereof. In the upper portion of the deep trench, the node dielectric serves as an etch stop layer during the formation of the buried-strap region. In the lower portion of the deep trench, the node dielectric separates the buried plate diffusion region from the deep trench conductor.
The node dielectric, which is composed of a conventional dielectric material such as a layered SiN/SiO structure, is formed by conventional processes such as thermal nitridation, CVD, plasma-assisted CVD, sputtering deposition, and the like. The deep trench is then filled with deep trench conductor 22 such as polysilicon and thereafter the deep trench conductor is recessed by conventional means to a depth which is desired for the strap (determines the channel length of the vertical MOSFET; typically of from about 100 to about 400 nm). The structure containing recessed deep trench conductor 22 is shown in FIG. 1(d).
At this point of the process, a strap process, such as described in Radens, et al. xe2x80x9cAn Orthogonal 6F2 Trench-Sidewall Vertical Device Cell for 4 Gb/16 Gb DRAMxe2x80x9d, IEDM 2000 Tech. Dig., p. 349, is employed in forming buried-strap outdiffusion region 24; see FIG. 1(e). In the exemplary structure shown in FIG. 1(e), a one-sided strap (OSS) process is employed to form straps facing each other. It is understood however, that other layouts may form straps such that they do not face each other or, are on both sides of the trench. That is, other types of vertical MOSFET3 cells may employ straps on more than a single sidewall of the trench. Note that the OSS process does not etch one of the collar oxide regions, and on that side of structure is the area in which body continuity 19 is achieved. Specifically, the buried-strap outdiffusion region is formed as follows: First, an oxide layer (not shown) is formed over the recessed deep trench conductor, followed by an etch stop liner which lines the node dielectric as well as the oxide layer previously deposited and thereafter a polysilicon placeholder material (not shown in the drawings) is formed in the upper region of the deep trench covering the exposed portions of the etch stop liner. Next, a portion of the polysilicon placeholder material on the side of the deep trench where the buried-strap is desired is etched down to the etch stop liner overlying the oxide layer at the top of the recessed conductive material.
An OSS process is then performed which may include the following processing steps: removing a portion of the polysilicon placeholder material using an etch process that is selective to the etch stop liner on a side of the deep trench where a strap is to be formed; removing the exposed collar oxide by utilizing an isotropic oxide etching process; removing portions of the etch stop liner and the node dielectric that are not protected by the remaining region of the polysilicon placeholder material; removing the remaining polysilicon placeholder material; opening a portion of the oxide layer over the deep trench polysilicon not covered by the etch stop liner; continuing the oxide etching so as to form a divot in the top collar oxide at approximately the top level of the deep trench conductor; and filling the divot with a conductive material such as doped polysilicon so as to provide a bridge between the deep trench conductor and the wall of the trench. During a subsequent annealing step, dopant from the divot filled region diffuses forming buried-strap outdiffusion region 24. The divot filled region is labeled as 26 in the drawings. Note that on the remaining wall portion of the structure not containing buried-strap outdiffusion 24 and divot filled region 26 is an xe2x80x9cintactxe2x80x9d collar oxide region 18, both directly beneath the divot filled region 26 and also on the wall not having a strap which extends all the way to the etch stop pad layer 12. The intact collar oxide serves to electrically isolate body region 19 from trench capacitor 22.
As is also shown in FIG. 1(e), trench top oxide (TTO) 28 is formed on all horizontal surfaces including deep trench conductor 22 and divot filled collar oxide region 26 utilizing conventional deposition processes such as high-density plasma-assisted deposition and thereafter a conventional resist recess process is employed to remove the trench top oxide from top surfaces of the structure. A sacrificial oxide layer (not shown) is next formed and stripped utilizing conventional lithography and etching and gate dielectric 30 such as an oxide is formed on the exposed walls of the upper portion of the deep trench. The gate dielectric is formed utilizing any well-known process such as thermal oxidation of silicon or by nitridation of an oxide and thereafter, a gate conductor 32 such as doped polysilicon is then formed on top trench oxide 28. The structure 32 is then planarized to hard mask 14 and thereafter, as shown in FIG. 1(f), the gate conductor 32 is recessed utilizing a conventional recessing process. Alternatively, a deposited gate dielectric, such as by CVD or ALD (atomic layer deposition), may be formed. The recessed gate conductor 32 is then capped with CVD oxide 34, using well known conventionally practiced processes.
It is the case that during the stripping of the sacrificial oxide layer at the exposed walls of the upper portion of the deep trench, portions of the TTO for protecting the collar and strap regions are at risk, with excessive etching possibly causing collar loss at the sides, edges and top, and moreover, loss of the doped polysilicon divot.
It would be highly desirable to eliminate collar oxide loss and loss of the doped polysilicon divot that may result from such sidewall etching.
It would be further highly desirable to enhance the degree of electrical isolation between the formed gate conductor and the capacitor electrode (deep trench conductor) in such a trench-capacitor vertical dynamic random access memory (DRAM) cell array device.
Additionally, it would be highly desirable to enable a thinner isolation region between the gate conductor and the capacitor electrode, to thereby assure the required overlap between strap diffusion and the gate conductor.
It is thus an object of the present invention to provide a TTO nitride liner for improved collar protection and polysilicon divot protection and TTO reliability during sidewall etch steps in trench-capacitor vertical dynamic random access memory (DRAM) cell array device processing.
It is another object of the present invention to enhance the degree of electrical isolation between the formed gate conductor and the capacitor electrode (deep trench conductor) in such a trench-capacitor vertical dynamic random access memory (DRAM) cell array device.
It is yet another object of the present invention to enable fabrication of a thinner isolation region between the gate conductor and the capacitor electrode (deep trench conductor) in such a trench-capacitor vertical dynamic random access memory (DRAM) cell array device, to thereby assure the required overlap between a strap diffusion and the gate conductor without compromising breakdown reliability.
It is a further object of the present invention to provide flexibility in the TTO deposition process.
In accordance with the teachings herein, there is provided a structure and method of forming a DRAM cell array comprising the steps of:
(a) forming a plurality of deep trenches in an array portion of a Si-containing substrate having at least a hard mask formed thereon, said plurality of deep trenches being arranged in rows and columns and including at least collar oxide regions formed on walls thereof and a recessed deep trench conductor formed between said collar oxide regions and defining a capacitor electrode for a DRAM cell;
(b) forming a buried-strap outdiffusion region within a portion of the wall such that the portion partially encircles the wall;
(c) forming a nitride liner layer above a horizontal surface of the deep trench conductor and enclosing exposed sidewall and collar oxide regions;
(d) depositing top trench oxide (TTO) layer above the formed nitride liner layer;
(e) performing TTO sidewall etch to remove TTO oxide which has been deposited on the vertical sidewalls and collar oxide, the nitride liner acting to protect the collar oxide layer from being etched;
(f) performing nitride liner etch to remove the portion of the TTO nitride liner which is exposed after TTO oxide removal; and,
(g) forming a vertical MOSFET by growing a gate dielectric on exposed walls of the deep trenches and forming a gate conductor above the TTO oxide layer within the walls of the deep trenches lined with the gate dielectric, wherein the formed TTO layer having underlying nitride liner eliminates possibility of TTO dielectric breakdown between the gate conductor and the capacitor electrode of a DRAM cell.
The deposition of a thin nitride liner just before TTO (Heavy Deposition Plasma) HDP deposition according to the invention functions to protect the collar oxide from being etched during the TTO oxide sidewall etch and provides additional lateral etch protection which is not realized in current processing schemes.
The process sequence does not rely on previously deposited films for collar protection, such as the Cut Mask (CM) nitride in the current processing schemexe2x80x94POR (Process of Record). This decoupling of TTO sidewall etch protection from previous processing steps provides additional process flexibility, such as allowing a thinner CM nitride and greater nitride etching during node nitride removal and buried strap nitrided interface removal. The sequence allows the complete removal of the CM nitride before buried strap poly deposition, thereby eliminating the possibility of trapped poly under the CM nitride along the remaining collar which is a serious concern in the current process.
Advantageously, the system and method of the invention eliminates the possibility of TTO dielectric breakdown between the gate and node electrode of the formed trench-capacitor in a vertical dynamic random access memory (DRAM) cell array device.